The present invention relates to a signal generator and a signal strength detecting circuit having the signal generator installed therein, and more particularly to a detecting circuit using a test tone signal to calibrate a transfer function of the detecting circuit.
A power detector is utilized for detecting power of an input signal, e.g., a wireless signal. Conventionally, magnitude of the detected power is expressed in decibels (dBVrms or dBm). If a wide detection range of input power is required, a linear-in-dB transfer function is preferred. FIG. 1 is a diagram illustrating the transfer function of an ideal power detector with linear-in-dB transfer function, where X-axis represents the power expressed in dBm of the input signal, and Y-axis represents the output voltage level corresponding to the input signal and expressed in V. It can be seen that, ideally, the relationship between the power in dBm of the input signal and the output voltage level corresponding to the input signal is a straight line having a predetermined slope. However, normally power detector circuit can only produce approximate transfer function to a real straight line, introducing so-called log-conformance error. This log-conformance error, together with process variation, temperature drift, and circuit mismatch effect of the power detector, make the relationship between the power in dBm of the input signal and the output voltage level corresponding to the input signal deviate significantly from the ideal straight line, as shown by the dashed line 12 in FIG. 1. Consequently, the output voltage level generated by the power detector may not correspond to real input power Vpr of the input signal, but to a deviated input power Vpd. Therefore, calibration of a power detector to obtain an accurate transfer function of the power detector becomes an important issue in this field.